High temperature- and corrosion-resistant material

ABSTRACT

The present invention relates to refractory, corrosion-resistant materials, suitable for the production of crucibles and the like, produced by compressing at elevated temperatures and pressures mixtures comprising 10-16 percent, by weight, of aluminum nitride and 90-40 percent, by weight, of boron nitride containing 5-25 percent, by weight, of boron oxide.

United States Patent 1191 Reinmuth 1 Dec. 17, 1974 [5 HIGH TEMPERATURE- AND 3,108,887 10/1963 Lenie et a1 106/65 CORROSION RESISTANT MATERIAL 3,718,490 2/1973 Morgan et a1. 106/65 [75] Inventor: Klaus Reinmuth, Durath, Germany [73] Assignee: Elektroschmelzwerk Kempten Primary Examiner.l. Poer GmbH, Munich, Germany Attorney, Agent, or FirmFrancis M. Crawford [22] Filed: June 27, 1973 App]. No.: 374,012

Foreign Application Priority Data July 7, 1972 Germany 2236437 US. Cl. 106/65, 106/734 Int. Cl C04b 35/58 Field of Search 106/65, 73.4

References Cited UNITED STATES PATENTS 6/1958 Taylor 106/65 ABSTRACT 2 Claims, N0 Drawings The present invention relates to refractory, corrosion-resistant materials comprising -60 percent, by weight, of aluminum nitride and 90-40 percent, by weight, of boron nitride containing 5-25 percent, by weight, of boron oxide, and to the production of said materials by compressing said mixture of ingredients at temperatures of the order of l,500-2,200C and pressures of the order of 30-2OO kg/cm It is well known in the prior art that the nitrides of such elements as aluminum, boron and silicon may be sintered into shaped articles having technically attractive densities and strengths with the aid of suitable binders (British Pat. No. 887,658).

It is also well known that boron oxide, calcium oxide, silicon dioxide, titanium dioxide, aluminum oxide, as well as other oxides, may be used as binders for boron nitride. The use of boron oxide, forexample, as a binder, however, while giving a shaped article having good densification, is not very refractory and absorbs water and hence products thus prepared are unsuitable for many purposes, such as crucibles.

Other types of previously suggested binders have likewise been found to be unsuitable for various reasons. For example, many of them require the use of impractically large proportions of the binder with the result that the particularly desirable properties of the boron nitride, such as its slipperiness, its easy machinability and its strength at high temperatures are lost.

It has now been discovered, in accordance with the present invention, that shaped refractory articles, substantially free from the above and other disadvantages, can be produced when the binding of the ingredients is effected with the new type of bonding agent disclosed following character resultszboron nitride, Al- O 2 B 0 aluminum oxynitride having the formula Al ,t,, ,O N, aluminum oxide and perhaps some other oxide/oxynitride mixed phases. This mixture of materials hereafter is simply referred to as binderf has a value between 0 and 4. cf ASTM x-ray data sheets Nos. 18-5] and l8-52.

As the reaction product obtained under the above conditions is corrosion-resistant and also resists high temperatures and temperature changes it is particularly suitable for use in the production, processing and testing of metals in the molten state.

The following specific examples will serve to illustrate the invention. it should be understood, however, that it is not limited to the specific temperatures or pressures set forth nor to any particular explanations of the reactions which-take place.

EXAMPLES in the hot pressing of the mixtures of aluminum nitride and boron nitride use was made of boron nitride power containing boron oxide finely distributed therewith (10 m /g BET-surface).* Preferably, use is made of boron nitride-powder which retains boron oxide from its method of preparation. Less favorable surface values are obtained when boron oxide is added to boron nitride which does not contain the specified amounts of boron oxide. However, such mixtures of boron nitride containing boron oxide may be used for the preparation of shaped articles according to the invention, even though the strength values will generally be somewhat lower than when the preferred mixture of boron nitride and boron oxide described above is used. BET-surface refers to powder surface increased by gas adsorption as described by Brunauer, Everett and Teller,

Table l below shows the properties of articles made in accordance with the above general procedure.

TABLE 1 Experiment No, I ll lll lV Raw material mixture "/1 by wt BN l57( B 0 80 20 BN (0.271 B 0 37 75 ALOg 10 micron smallest) 2O AlN micron smallest) 2O 8 5 Mol. ratio AlN/luO in mixture 2.8 1.6 2,8 Sintering temp 2l00 2100 2100 2100 Sintering pressure kp/cm lOO I00 I00 100 Density 7! 83 82] 75* 75* Transverse rupture strength kp/mm 6.l** 3.2** 3.2 042* Hardness lip/mm 10.4 7.8 7.5 2.4 Ratio bendin strength vertical/parallel 1.9/1 7/1 4/1 2/1 Ratio therma expansion vertjparallel l/l.2 l/55 Ratio thermal conductivity vertJparallel 1/1 5/] Signifies the maximum density which can he produced (maximum compacting) This represents the bending strength measured with samples which were cut vertieal to the direction of pressing that during the reaction between the aluminum nitride 6 and the boron oxide phase presumably nitrogen and oxygen at least partially become exchanged so that, depending upon the sintering conditions, a mixture of the On the basis of X-ray diffraction patterns it will be noted that the products of experiments I and ll consist of aluminum oxide and boron nitride. However, in spite of having the same chemical compositions the product of experiment I shows better mechanical properties. Furthermore, this material may become still more densifted.

A significant difference between the products of experiments l and ll consists in the striking tendency of the product from experiment II to orient the layer planes of boron nitride vertical to the direction of pressing. This orientation, which occurs at the boron nitride sintering, makes itself felt in the values of some properties which show a directional dependency, like the transverse rupture strength, the thermal expansion and the thermal conductivity.

in producing the product of experiment ill the aluminum nitride assay was decreased and simultaneously the molar'ratio AlN/B O became shifted in favor of the boron oxide by the addition of boron nitride poor in boron oxide. The resulting product, which still has an impressive transverse rupture strength, shows a more riod of 20 hours and were not attacked by it during this period. The product of experiment Vl retained its shape when stored in liquid aluminum for a period of 180 hours. The corrosive attack in a cryolite bath in all three experiments was smaller than 0.04 mm per hour. The articles containing some aluminum oxide in the bonding agent did not dissolve faster than an article made from pure aluminum nitride plus boron nitride.

The products from experiments V and VI, in spite of pronounced orientation due to the excess of B 0 lf 1() the g uminum 6011mm and in Spite of the hard the aluminum nitride assay is lowered still more, as in the case of experiment IV, the strength drops still more. This indicates that if it is desired for the boron nitride to become practically solidified without any orientation by the reaction products from aluminum nitride and boron oxide, it is necessary to use at least 10 percent, by weight, of aluminum nitride in the reaction mixture and at least two mols of aluminum nitride per mol of boron oxide.

In the experiments shown in Table ll below the starting mixtures contained 50 percent, by weight, of aluminum nitride and the products again showed better strengths when the bonding agent of the present invention was used.

bonding agent could still be processed with chipremoving tools, it thus being possible to produce from them articles of any desired shape needed in the production and processing of aluminum, as well as in various branches of metallurgy and in chemical technology. With significantly higher assays of aluminum nitrides the products of the experiments could no longer be readily machined with chip-removing tools.

What is claimed is:

1. Process for the production of refractory, corrosion-resistant articles which comprises compressing at temperatures of the order of l,500-2,200C and pressures of the order of -200 kg/cm a mixture of 10-60 percent, by weight, of aluminum nitride and 90-40 per- The product of experiment V has been sintered so as to give the highest obtainable density. The product of experiment Vl could be ground down on rotating discs and became nicely polished, whereas the product from experiment Vll, which did not contain the assay of boron oxide required by the present invention, showed poor texture and ruptured grains, even though the arti-. cles of both experiments VI and Vll showed the same density.

The products from experiments V, VI and Vll were not wetted by liquid aluminum at 900C during a pecent, by weight of boron nitride containing 5-25 percent, by weight, of boron oxide.

2. Process for the production of refractory, corrosion-resistant articles which comprises compressing, at temperatures of the order of l,500-2,200C and pressures of the order of 30-200 kglcm 10-60 percent, by weight of aluminum nitride with 90-40 percent, by weight, of boron nitride containing 5-25 percent, by weight, of boron oxide, at least two mols of aluminum nitride per mol of boron oxide being present in said mixture. 

1. PROCESS FOR THE PRODUCTION OF REFRACTORY CORROSIONRESISTANT ARTICLES WHICH COMPRISES COMPRESSING AT TEMPERATURES OF THE ORDER OF 1,500*-2,200*C AND PRESSURE OF THE ORDER OF 30-200 KG/CM2 A MIXTURE OF 10-60 PERCENT, BY WEIGHT, OF ALUMINUM NITRIDE AND 90-40 PERCENT, BY WEIGHT OF BBORON NITRIDE CONTAINING 5-25 PERCENT, BY WEIGHT, OF BORON OXIDE.
 2. Process for the production of refractory, corrosion-resistant articles which comprises compressing, at temperatures of the order of 1,500*-2,200*C and pressures of the order of 30-200 kg/cm2, 10-60 percent, by weight of aluminum nitride with 90-40 percent, by weight, of boron nitride containing 5-25 percent, by weight, of boron oxide, at least two mols of aluminum nitride per mol of boron oxide being present in said mixture. 